Two staged spontaneous magnetic field generation in direct drive ICF coronae

Magnetic fields can be self-generated during ICF implosions through a variety of mechanisms, and may modify hydrodynamic instability evolution as well as hot electron transport. However, the precise evolution of experimentally observed field structures is relatively poorly understood. To probe these fields, target-normal sheath acceleration proton radiographs were obtained of 60-beam exploding pusher implosions at OMEGA. Two distinct stages of magnetic field self-generation are observed, corresponding to dominant wavevectors of the magnetic field structures that are initially parallel, and then perpendicular to, the radial direction. These magnetic field structures are attributed to expansion-driven and temperature-gradient driven Weibel instabilities. Particle-in-cell simulations and synthetic proton radiography are used in concert with the experimental proton radiographs and magnetic field reconstructions to verify the expected magnetic field topologies and assess the magnetic field strengths within the implosion timescale.

This work was supported in part by the U.S. DOE and NLUF.